Abbe number is a measure of color dispersion or chromatic aberration, which is caused by the fact that refractive indices of materials are wavelength dependent. Abbe number is generally defined as (n.sub..lambda.v -1)/(n.sub..lambda.1 -n.sub..lambda.2) where n.sub..lambda.1 and n.sub..lambda.2 are the refractive indices at two different wavelengths, and n.sub..lambda.v is the refractive index at a median wavelength, i.e., yellow. Ophthalmic (eyeglass) lenses made of glass or plastic and having Abbe numbers ranging from about 25-60, are generally relatively free of annoying color dispersion. Abbe numbers in the range of 40-60 are especially desired, because an ophthalmic lens (e.g., -4.00 D, 75 mm in optic diameter) with an Abbe number of 40 will produce 10 mm of image separation.
The angular separation between two different wavelengths increases with lens power. Generally, the color dispersion, is high for plastic materials which have refractive indices above 1.50. These materials often possess aromatic moieties with extended conjugation and high polarizability, with electronic absorption in the near ultraviolet wavelength range, which contribute to the refractive index. However, aromatic groups possessing electronic absorption bands near the visible wavelength range (400 nm-700 nm) generally lead to an increase in color dispersion of the material. An example of such a high refractive index material is polycarbonate of bisphenol A, which has a refractive index of about 1.58 and possesses other desirable properties, such as superior impact resistance. The Abbe number for this material is quite low, for example 28-32, depending on the end groups and processing methods. Therefore, while polycarbonate of bisphenol A is widely used as a material of choice for safety glasses, its use is largely limited to medium to low prescriptions because higher prescriptions cause an increase in color dispersion, and therefore lead to undesirable color separation in white images.
Prescriptions higher than +/-4.00 D generally require the use of other high index materials which are generally more costly than polycarbonate and do not have as good an impact resistance as polycarbonate. These high index materials have been developed with an improved color dispersion, but even the best of these high index materials, that is having a refractive index greater than 1.57, have Abbe numbers in the range of 30-50. The color dispersion of such materials is generally only marginally acceptable. People wearing eyeglasses made of these materials often experience chromatic aberration, especially at the periphery of their visual field, since off axis rays are more prone to chromatic aberration than axial rays close to the optical center.
The present invention includes a compound ophthalmic lens and a method of fabricating such a lens which permits the use of plastic or glass materials having a high refractive index and impact resistance, such as polycarbonate of bisphenol A or other materials, in making eyeglass lenses without sacrificing satisfactory integration of colors in the image, as measured by Abbe numbers in the range of about 40-100.
Accordingly, it is an object of the present invention to provide an ophthalmic lens with an Abbe number in the range of about 40-100, and preferably in the range of about 60-80.
It is another object of the present invention to provide a method of fabricating such a lens using a desired material having a high refractive index and impact resistance, such as polycarbonate of bisphenol A and diethylene glycol bisallyl carbonate (CR-39.TM.), for fabrication of ophthalmic lenses.